No, we can't.
You need to qualify things: how long is "too long"? Have you measured it?
Use the Stopwatch class to accurately time what is going on - and then at least you have a measure of what you are doing and how long it takes so you can check how any changes you make affect that time. There is a tip here that is concerned largely with timing things: Counting lines in a string
] have a look at how it is done, then start working out why your code is so slow. (I suspect I know, but that is just a guess so I won't share it at this time - a quick test will tell you which bit of your code is taking most of the time.)
BTW: Don't do it like that - do not concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Use Parametrized queries instead.